Field of the Invention
The present invention relates to image processing in tomographic image diagnosis using radiation.
Description of the Related Art
In recent years, in the radiation imaging field, tomosynthesis that detects, by a detector, radiation (X-rays) emitted at limited angles to obtain projection images, and reconstructs a tomographic image using the projection images is attracting attention. Tomosynthesis has an advantage that it can be readily applied to a fluoroscopic table, a general imaging table, a mammography apparatus, and the like without requiring a large-scale apparatus.
FIG. 1A is a schematic view showing chest imaging in a system that reconstructs a tomographic image by tomosynthesis. In tomosynthesis, projection images are captured by changing the position and irradiation angle of an X-ray tube as indicated by 101 to 103 in FIG. 1A, and also translating the position of an X-ray flat panel detector (to be referred to as an FPD hereinafter) in a direction opposite to the moving direction of the X-ray tube as indicated by 104 to 106. Note that the FPD does not move in some cases.
At this time, a point 109 at which the focal point of the X-ray tube intersects a plurality of beam axes passing through the center of the FPD is called an isocenter, and a sectional image passing through this point is clearest. Therefore, it is common practice to perform imaging by setting the isocenter 109 at the height of a portion of an object 107 to be diagnosed. However, an isocenter section 108 as a plane passing through the isocenter 109 is a plane on which a clearest tomographic image can be obtained but artifacts readily occur. The reason for this will be explained by exemplifying a shift addition method as the most basic reconstruction method of tomosynthesis.
The shift addition method is a method of obtaining a tomographic image by adding, to a sectional pixel value, the projection pixel value of a point at which a straight line connecting the focal point of the X-ray tube and a point of a section intersects the FPD. In this method, to reconstruct a point 113 on the isocenter section shown in FIG. 1B, the pixel values of points 110 to 112 on the FPD are added. Since the FPD moves as indicated by 104 to 106 along with movement of the X-ray tube indicated by 101 to 103, these points correspond to a pixel at the same position on the FPD. As a result, the tomographic pixel value of the isocenter section is obtained by adding the pixel values at the same position on the FPD at all angles (that is, by simply adding all projection images). Therefore, on the isocenter section, random noise that temporally changes is largely suppressed but fixed patterns that do not temporally change and that are caused by the FPD are emphasized.
The fixed patterns include, for example, small dirt or flaw on the surface of the FPD, the boundaries of divided driving circuits inside the FPD, the boundaries of divided substrates, and a small pattern existing in a dark image. The fixed patterns are corrected by gain correction using an X-ray image captured when no object exists, dark correction using a dark image obtained when no X-ray irradiation is performed, or the like to a level at which they do not stand out in projection images, thereby shipping the system. However, a small pattern may remain due to a difference in irradiation conditions of the X-ray tube, temporal deterioration, a change in driving load of the FPD, or the like. As a result, even if the fixed patterns do not stand out in projection images, artifacts on the isocenter section may appear at the time of reconstructing a tomographic image for the above-described reason.
Note that in tomosynthesis, a new reconstruction method such as FBP (Filtered Back Projection) or successive approximation reconstruction can be used. However, in these methods, the basic principle is the same as that of shift addition (shift addition is equivalent to CT back projection), and stronger artifacts may appear on the isocenter section due to the high-frequency emphasis effect caused by filter processing, iterative processing, or the like.
Since the isocenter section is a section on which a clearest image is obtained and which has high diagnostic value, the occurrence of artifacts degrades drawing of a lesion or organ on a tomographic image, thereby significantly reducing the diagnostic performance of the tomographic image.
Random noise is superimposed on projection images by X-ray photon noise or system noise caused by the apparatus. As a result, the tomographic image includes random noise. To cope with this, as a method of reducing random noise, U.S. Pat. Nos. 8,416,914 and 8,005,286 describe methods of reducing random noise by providing the “smoothness” of a tomographic image a priori in successive approximation reconstruction of the tomographic image. The techniques described in U.S. Pat. Nos. 8,416,914 and 8,005,286 reduce random noise by considering the energy (or its function) of the random noise at the time of successive approximation reconstruction. However, these methods do not consider the energy of fixed patterns such as the above-described artifacts on the isocenter section.
The present invention effectively reduces artifacts on an isocenter section.